The work proposed in this application is directed toward the correlation of the structure and function of the proteins of the complement system. The areas which are presently under investigation are: 1. The role of sugar moeities in the function of the Clq subunit of the first component of complement. 2. Structural relationship between the fourth component of complement (C4) and its serum precursor (pro-C4). 3. The mechanism of action of a new control protein of the complement system (C4bp) which binds to the activated fourth component of complement (C4bp). The role of this protein in the formation and function of the classical pathway C3 convertase is under investigation. 4. The mechanism of complement activation by IgA immunoglobulins is being studied using immune-complexes prepared with mouse IgA1 and IgA2 with specificity to phosphorilcholine.